In many energy-storage systems, the volume available for gas storage is restricted and, indeed, can be a limiting factor for a given technology. For example, the acceptance of natural gas as a transportation fuel has been slow mainly due to the lower energy density of that gas as compared with gasoline. The same is true regarding hydrogen and hydrocarbon gases (e.g., methane) for the use in spacecraft propulsion systems and fuel cells. The storage of hydrogen is certainly a critical barrier that needs to be overcome before fuel cells can be widely used in transportation applications. There exists therefore a strong need for the development of lightweight, compact, high energy density storage of gases for a variety of applications.
At present, there are two principal approaches to this problem: (a) physical storage (compressed gas, liquefaction, and adsorption); and (b) chemical storage (e.g., metal hydrides for hydrogen storage). The foregoing approaches have however the following limitations: Compressed gas systems have high weight-to-volume ratios due to the heavy containers required for the storage of pressurized gas. Vessels made from lightweight graphite composites show some promise in alleviating this difficulty, but further advancements and product development are required to assure safe and reliable use of these materials. Cryogenic storage carries with it significant penalties because of the energy required to liquefy the gas and to maintain it in a liquefied state. Metal hydrides, or other suitable chemicals, remain a possible option, but low-cost hydrides currently require high temperatures to liberate the stored hydrogen; moreover, the hydrides which are capable of liberating hydrogen at low temperatures have a very low storage capacity.
Gas storage based on adsorption on activated carbon offers large storage capacity in terms of weight (2-4 times greater than state-of-the-art hydrides). Activated carbons have a great potential to provide substantially larger storage capacities than gas compression because the density of the adsorbed phase is greater than gas density. The high density of gas at the surface more than compensates some loss of the available volume associated with the introduction of a sorbent. Commercial activated carbons have a serious limitation, however. A large portion of their porosity resides in mesopores (pore size, dp=2-50 nm), whereas it is the microporosity (dp less than 2 nm) that is most desirable for high storage capacity. This can be rationalized by the presence of stronger sorbent-adsorbate interactions within small pores. Thus, to maximize gas storage capacity it is necessary to develop a method of fabricating highly microporous carbons.
A key to successful gas-storage sorbents is the ability to xe2x80x9cengineerxe2x80x9d the pore structure in such a way so that only pores having optimum dimensions are formed. The optimum dimension will, generally speaking, be different for different gases, but the common features of good gas-storage sorbents are: (1) a large number of pores with a narrow pore-size distribution within the micropore region (pores smaller than 2 nm); (2) the absence of meso and macroporosity (pores larger that 2 nm); and (3) minimization of void space within the storage container (consistent with the form of the sorbent employed).
In general, it is difficult to satisfy all the above criteria simultaneously. A significant challenge is often associated with increasing the specific surface area of carbon (by char activation) in such a way so that pores of uniform size are formed throughout the entire volume of a large, monolithic sorbent element (e.g., a disc-like element having a minimum dimension that is on the order of one centimeter). Char activation is usually carried out by reacting carbon with an oxidizing agent, typically steam or carbon dioxide, but the use of oxygen is also possible. During the activation of large sorbent elements, pores close to the surface of the sorbent react with the oxidizing agent before the pores inside the sorbent have a chance to interact with the oxidizing agent. In other words, diffusion of the activating agent through the porous medium is slower than the rate of reaction between the gas molecules and carbon. As a consequence, the parts of the pores that are close to the surface react preferentially and get enlarged, whereas the porosity deeper within the sorbent remains underdeveloped. This leads to the non-uniform evolution of porosity and pore-mouth widening, which are highly undesirable for gas-storage sorbents.
In an effort to resolve the above problem, an innovative char-activation process was developed (Quinn and Holland, U.S. Pat. No. 5,071,820, 1991; Wxc3x3jtowicz, M. A., Smith, W. W., Serio, M. A., Simons, G. A., and Fuller, W. D., xe2x80x9cMicroporous carbons for gas-storage applications,xe2x80x9d Proc. Twenty-Third Biennial Conference on Carbon, the Pennsylvania State University, Jul. 13-18, 1997, vol. I, pp. 342-343; Wxc3x3jtowicz, M. A., Markowitz, B. L., Smith, W. W. and Serio, M. A., xe2x80x9cMicroporous carbon adsorbents for hydrogen storage,xe2x80x9d International Journal of the Society of Materials Engineering for Resources 7 (2), 253-266, 1999). The technique is called cyclic chemisorption-desorption activation and involves cyclic repetition of the following two steps: (1) oxygen chemisorption on carbon surface at a relatively low temperature Tch for a period of xcfx84ch to form stable oxygen surface complexes; and (2) high-temperature (Td) desorption of the surface complexes as carbon monoxide, which is carried out in the absence of oxygen. The desorption step is carried out for a period xcfx84d. Both the low temperature of the chemisorption step and the inert atmosphere of the desorption step prevent excessive burn-off and the creation of larger-size pores via pore-mouth enlargement. In each chemisorption step, this method allows deep penetration of the oxidizing agent into the farthest and smallest pores, without the preferential burn-off of the carbon material at the pore mouth.
The broad objects of the invention are to provide a novel method for the preparation of sorbents for the high energy density storage of gases, particularly hydrogen; to provide novel sorbents so produced; and to provide novel gas storage units employing such sorbents.
More specific objects are to provide such a method, sorbent and unit wherein the sorbent constitutes a highly microporous carbon element having an improved pore structure and gas adsorption capacity.
It has now been found that certain of the foregoing and related objects of the invention are attained by the provision of a method for gas storage, and particularly for the storage of hydrogen, comprising the steps: (a) effecting carbonization of a carbonaceous precursor material in a substantially nonoxidizing atmosphere at temperatures that attain an upper value (Tf) of at least about 1000xc2x0 C. and for a residence (hold) time (xcfx84f) of determined duration at that temperature (which may be momentary); and (b) introducing the gas to be stored, under positive pressure, into a storage vessel containing a substantial amount of the sorbent material so produced. It is believed that a substantial amount of the carbon content of the precursor material is converted to a graphitic form in the carbonization step.
In certain embodiments, the upper value of temperature attained in the carbonization step will be at least about 1100xc2x0 C., and the carbonization step may advantageously be effected by maintaining the precursor material in the nonoxidizing atmosphere, and at the upper temperature value attained, for a period of at least about one minute and, under certain circumstances, preferably about 30 minutes. The process for producing the sorbent material will preferably include at least one activation step effected subsequent to the carbonization step, which activation step will advantageously comprise multiple cycles of chemisorption and desorption.
Other objects of the invention are attained by the provision of a sorbent material produced in the manner described, and still other objects are attained by the provision a gas storage unit comprising: a vessel; a substantial quantity of the described sorbent material contained in the vessel; and a quantity of gas adsorbed by the sorbent material. To obtain optimal storage capacity, the gas will advantageously be introduced into the storage vessel at a pressure in the range of about 500 psi to 3500 psi, and the vessel will contain at least about 75 percent of the sorbent, based upon its volumetric capacity.